Plastic deformation by rolling, extrusion and drawing often increases the strength of metal alloys, but decreases their ductility [1]. By contrast, processing metals and alloys by severe plastic deformation (SFD) can increase their strength while maintaining good ductility by forming ultrafine grains (UFGs), and subgrains, from smaller than 100 nanometers (nm) to about 1000 nanometers [2]. The combination of high strength and good ductility makes SPD-produced ultrafine-grained (UFG) materials very attractive for medical implants [3], aerospace structures, sporting goods, automobile parts and other devices.
Among the SPD techniques, “equal channel angular pressing” (ECAP), also known in the art as “equal channel angular extrusion” (ECAE™), has attracted much attention because it is very effective in producing UFG structures and can produce UFG billets that are large enough for practical structural applications [4]. Only High Pressure Torsion (HPT) [5] is more effective in producing UFG structures. However, HPT can only produce small disks with a typical diameter of about 10 millimeters (mm) and a thickness of less than about 1 mm. These dimensions make them unsuitable for most structural applications. By contrast, ECAP has been used to produce billets that are long enough and wide enough for some practical structural applications.
The original ECAP technique involves pressing a workpiece through a die with two channels that are equal in cross-section and intersect each other at an angle. Sending the workpiece through the die refines the microstructure, and when the die cross-section is circular or square shaped, the workpiece can be turned 90 degrees and extruded again and again because the shape and size of the workpiece does not change substantially during the pressing.
The ECAP technique in its original design has some limitations: the aspect ratio (i.e. the length to diameter ratio) of the workpiece must be smaller than a critical value so that the workpiece does not bend during the pressing, and the ram that forces the workpiece through the die has a limited travel distance. These aspects of the ECAP technique place limits on the length of the workpiece and make ECAP a discontinuous process with low production efficiency and high cost. In addition, a significant length near each end of a workpiece is usually cracked and has to be removed, wasting a significant portion of the workpiece and further increasing the cost of the product. The discontinuous nature of ECAP and the wasted portions of the processed workpiece make UFG products expensive, which limits their applications to high-valued markets such as medical implants and devices where the cost of the materials is a relatively minor portion of the total cost. A key to commercializing the preparation of UFG materials is to lower their processing cost and minimize waste through continuous processing.
In the early 1970's, Green and Etherington developed an effective process, now known as the CONFORM™ process, which is directed to continuous rotary extrusion that converts powder feedstock into a long solid article [6]. Briefly, a CONFORM™ apparatus includes a disk and a shoe that provide frictional force to drive feedstock through the apparatus. Feedstock is sent through a channel formed in between the disk and the shoe. A groove in the disk covered with the stationary shoe forms the channel, and the contact interface between the feedstock and the shoe results in dragging frictional force. The feedstock has three interfaces driving it forward and one interface dragging it backward, with a net forward driving force. An abutment on the inner surface of the shoe stops the feedstock and forces it through an outlet. The outlet cross-section usually has a different shape from the groove because the objective of CONFORM™ is to change the geometry of the feedstock (and consolidate the feedstock if powder feedstock is used), which usually requires only one pass. The deformation of the feedstock during extrusion is similar to a conventional extrusion process.
Another continuous method called “repetitive corrugation and straightening” (RCS) has been used to process metal sheets and rods in a continuous manner [7]. RCS is less effective at refining grains than ECAP is, and each RCS pass produces non-uniform strain along the length as well as the thickness of the workpiece.
A coshearing process [8] and a “continuous constrained strip shearing (C2S2) process” [9] were recently reported for continuously processing thin strips and sheets. Both processes use the friction created between the rollers and the workpiece to push the workpiece through a modified ECAP die. The former [8] uses several rollers to increase the frictional force, while the latter uses one set of rollers but employs workpiece thickness reduction to increase the frictional force. Both are limited to processing sheet metals because the frictional force required to push the workpiece through the ECAP die is proportional to the contact area between the workpiece and the rollers, and only a workpiece in sheet form can provide enough frictional force. To process a workpiece in the form of a rectangular bar, more frictional force is needed to push the workpiece through an ECAP die.
No continuous process or apparatus thus far can refine the grain size of a rectangular bar without significantly affecting the cross section. There remains a need for an apparatus and process for the continuous processing of rectangular bars to refine the grain size without substantially affecting the cross section.
Therefore, an object of the present invention is to provide an apparatus for the continuous equal channel angular pressing processing of a rectangular bar workpiece without substantially affecting the cross-section.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.